Reintroduction of lignin-modifying enzymes in the brown-rot fungus Gloeophyllum trabeum restores ligninolytic activities but diminishes lignocellulose degradation.

Fungi have evolved two distinct strategies, white-rot and brown-rot, to degrade lignocellulose in plant biomass. White-rot fungi utilize lignin-modifying enzymes (LMEs) to deconstruct lignin and access carbohydrates, whereas brown-rot fungi, which arose from white-rot ancestral lineages, have largely lost LME activities. Instead, brown-rot fungi rely on a small redox metabolite-mediated Fenton system that generates reactive oxygen species (ROS) to rapidly deconstruct lignocellulose and selectively remove carbohydrates. The abandonment of LMEs in brown-rot fungi suggests an intriguing evolutionary strategy to streamline decay machinery, but it remains unclear why fungi evolved this way. Here, we reintroduced LME genes from the white-rot fungus Trametes versicolor into the model brown-rot fungus Gloeophyllum trabeum to create hybrid fungal systems and evaluate LME function within the Fenton-dominant brown-rot context. Analysis of lignocellulose degradation showed that lignin disruption can be partially restored in LME mutants, thereby shifting the typical carbohydrate-selective decay mode of G. trabeum. However, constitutive LME expression also caused pronounced growth defects and reduced the overall rate of lignocellulose degradation, specifically decreasing cellulose and hemicellulose removal. Correlation analysis between genotypes and phenotypes indicates that LMEs, although prevalently used for delignification in fungi, are fundamentally incompatible with the carbohydrate-selective brown-rot system.